Inkjet printers print dots on a print medium by ejecting small drops of ink from one or more nozzles. In order to ensure consistent color reproduction in an image built up on the print medium from a large number of drops of ink, it is important that portions of the image printed by different nozzles have equivalent colorimetry. This can be achieved by ensuring that the amount of ink present in each drop is the same for each drop ejected from all of the nozzles. Alternatively, for print elements that are found to eject smaller drops, the average number of drops ejected by that print element may be increased.
However, the amount of ink present in each ejected drop may vary due to slight differences between nozzles, due to changes in the ink being used, or for other reasons. A calibration process may be performed at regular intervals in order to determine colorimetry of the printer output as compared to a desired reference. The calibration process allows the control signals provided to each nozzle, or portion of a printhead to be adjusted such that the output is corrected for any variation that has occurred. This may be achieved by modifying the control signals directly based on calibration parameters, or by altering the image data to be printed to take account of variations in the response of the nozzles being used to print the image.
While regular re-calibration of the nozzles in an inkjet printer can ensure consistency for colors printed across a print medium, and for a series of print media, performing the calibration process wastes a certain amount of print media and ink. Furthermore, it has been found that requiring a user to perform regular calibration of the inkjet printer gives the user a perception that the printer may be unreliable.